I installed Defraggler on 2 different 2003 server.
I noticed that when it defrag a system or database file (in use) when he try to write it, just disappear.
I also installed degraggler on some client: if the user have administrative account on the machine it works fine.... id the user is a standard use (without administrative priviledge) windows signal a system error.... if you ignore it defraggler just work.